Welcome to the Ranch
Don't
ask the same question twice; I have deleted the other copy. If you confirm there is a difference between the two copies, I can find the deleted post.
I am afraid I can see all sorts of things wrong with that code.
1: Avoid making a display Component a Listener. The panel oughtn't to implement a Listener interface (except where the event relates directly to that component). Create a Listener class and add its instances to the three sliders. For some Listeners, it is possible to use a λ expression instead (Java8+ only).2: Why have you got a List? Why have you used a raw type? Why have you used Vector, which nobody uses any more? Where are you adding anything to that List? Why are you cloning that List?3: Why do you have a synchronized block? Since you mustn't access Swing components from multiple Threads, that looks unnecessary to me.4: Where do the colour listener and colour event interfaces come from? I am sure that isn't the correct way to change colours. And you appear not to be creating any instance of those two interfaces, anyway.5: Where are you using the binding group?I suggest you start by not using NetBeans GUI builders until you have lots more experience with Swing. I also suggest you don't need to extend any display classes, nor do you need that List in this class. Having a List there confuses the meaning of the panel: you are using it for business logic whereas display Components ought to be used for display only. If you do use a List, use an ArrayList. I also think you don't need the two interfaces at the bottom.
You are, I think, asking the wrong question. You do not want to make the colour available to different components. You want to tell the components to change to a particular colour, in this case the same as the current panel. Getting a colour is really easy:
I think it is not necessary to take a defensive copy, but I am not certain. In fact you will probably find that a
JPanel already has a method like getBackground() which does exactly that, so you don't need to write any methods here. Yes,
here is that method. But that won't sort out your problem. Notice that you don't need to have a colour field either, because there already is one in the panel, called background.
I think you need an object which encapsulates all the Components whose colours you want to change, so that entails a new class, with these methods:-
I also think you want a Listener for each slider.
ChangeListener is the correct listener to use, I think. It is a
Functional Interface so you can create a λ with it (Java8+ only): read about λs in the
Java™ Tutorials.
That λ is the equivalent of adding an instance of the same class to each of the three sliders, which is what I recommended at first.